Chemical synapses are specialized cellular junction structures between neurons and their synaptic partner cells. Synapses are essential means of communication between neurons in central nervous systems. During development, synapse formation is the ultimate step in wiring the nervous system. Both anatomical and physiological evidence suggest that synapses formed between neurons in local circuits are specific and stereotyped. It is not understood what molecular mechanisms underlie the synaptic specificity: the ability of synaptic partners to distinguish each other from other non-target contacting neurites. In the long term, we would like to understand the nature of specificity mechanisms and how these mechanisms lead to the assembly of functional synapses. Our previous work in C. elegans has established a genetic system to study synaptic specificity. We labeled a specific set of synapses from a motor neuron, HSNL. We asked how the synaptic partners of HSNL were selected and how the localization of synapses was determined. We discovered that two transmembrane immunoglobulin superfamily proteins, SYG-1 and SYG-2, were essential to determine the target specificity of HSNL. In both syg-1 and syg-2 mutants, HSNL fails to synapse onto its normal postsynaptic targets. Instead, ectopic synapses are formed onto abnormal targets. SYG-1 functions cell autonomously in the presynaptic neuron HSNL and localizes to synaptic sites at early stage of synapse formation. SYG-2 functions in the guidepost cells as a ligand of SYG-1 to cluster SYG-1 to synapses. SYG-1 and SYG-2 directly bind to each in biochemical assays. Therefore, interaction between two IgSF proteins defines synaptic specificity in HSNL. In this grant, we propose to understand how the SYG-1 and SYG-2 interaction leads to synapse formation and synaptic target selection. We will perform structure-function analysis on SYG-1 and SYG-2. We will test if this interaction is sufficient to trigger synapse formation and target selection. We will expand our genetic analysis to identify more molecular players in synaptic specificity by characterizing and cloning syg-3 and other specificity mutants. We will study the role of SYD-2 in synaptic vesicle clustering to understand how specificity mechanisms lead to the assembly of synapses. How synaptic specificity is achieved is a fundamental question in developmental neurobiology. Understanding this question should provide insights into how functional neuronal circuits are constructed during development, will potentially lead to new therapeutic strategies against nerve injuries and neurodegenerative diseases.